The present invention is directed to semiconductor device assembly and, more particularly, to a lead frame with corner tie bars used in the assembly process.
Semiconductor device packaging fulfills basic functions such as providing electric connections and protecting the die against mechanical and environmental stresses. An assembled semiconductor device has exposed electrical contacts and may be mounted on a support, such as a printed circuit board (PCB), for example, where the exposed electrical contacts are connected to external electronic circuits on the support. Using surface mount technology, the exposed electrical contacts of the device can be soldered directly to corresponding electrical contact pads on the support, providing mechanical attachment as well as electrical connections.
Semiconductor devices are commonly packaged for surface mounting by encapsulating one or more semiconductor dies in a mold compound. The encapsulation process embeds the die or dies within the molding compound. Various techniques are available for connecting the exposed electrical contacts of the device internally with electrical contact pads on the embedded semiconductor die, e.g., wire bonding.
The semiconductor device commonly has an electrically and thermally conductive metal flag (also called a die pad or paddle), which assists in cooling the device when it is being used (i.e., in operation), whether or not the flag is exposed at the surface of the encapsulation, The flag also may provide an electrical ground connection to the semiconductor die. It is common to facilitate manufacturing operations by performing many of the operations on an array of the semiconductor dies mounted on an array of flags that are linked together, the links being severed during a singulation operation. The links are typically provided by a frame structure, i.e., a lead frame array, which has an array of the flags connected by tie bars to frame members that are removed by being cut off and discarded during the singulation process. The frame structure also includes sets of the exposed electrical contacts that are supported by and integral with the frame members, until the devices are encapsulated and the frame members removed during singulation to isolate the electrical contact surfaces or leads from each other. This technique is applicable to devices where the sets of electrical contacts are disposed at the periphery of the flag and the semiconductor die, on two opposite sides or around all four sides.
In one type of surface mount semiconductor device, the flag is exposed at its bottom face but in another type the flag as well as the die are embedded in the encapsulation. In one type of package, known as Quad Flat No-lead (QFN), the exposed contacts are positioned in the bottom face of the encapsulation at its edge surface. In another type of package, known as Quad Flat Package (QFP), the exposed contacts are leads that project outward from the edge surface of the encapsulation.
In a wire bond package, the back face of the semiconductor die is mounted on a flag and the contact pads of the semiconductor die on its active face are connected to the exposed electrical contacts of the package with bond wires. A concern is to reduce movement of the bond wires due to flow of the liquid or semi-liquid molding compound during encapsulation (known as wire sweep). Such movement can result in excessive proximity of adjacent bond wires, resulting in excessive or variable mutual inductance, or even short circuits between two bond wires.
Continued reduction in the size of semiconductor devices and increases in their complexity results in an increase in the number of exposed electrical contacts of the devices and a reduction in the spacing between the electrical contacts and the spacing between the bond wires. It would be desirable to have more exposed electrical contacts, yet also to reduce the effects of wire sweep.